1. Field of the Invention
The present invention relates to a method and device for controlling the temperature of a heating source of an image forming apparatus such as a printer or a copying machine.
2. Description of the Related Art
In general, a fixing device has been used to increase the stability of an image produced by an image forming apparatus such as an electrophotographic printer. The fixing device, which includes a heater, fixes a toner image on a recording medium, and the surface temperature of the fixing device is an important factor for determining the fixing ability of a toner image on the medium. As the surface temperature of the fixing device increases, the image is more thoroughly fixed. However, it is inefficient to continue supplying energy to maintain the surface of the fixing device at a high temperature between printings, i.e., when the printing apparatus is in the waiting state. Thus, the surface temperature of the fixing device must be controlled depending on the printing operation.
FIG. 1 schematically shows the structure of a conventional image forming apparatus. Referring to FIG. 1, a photosensitive belt 100 rotates and moves along a path determined by first, second and third rollers 111, 112 and 113. A charger 120 for electrically charging the photosensitive belt is provided on one side of the photosensitive belt 100. A plurality of laser scanning units 130 irradiate a laser beam onto the photosensitive belt depending on image information to form an electrostatic latent image. A plurality of developing devices 140 coat a developer containing toner of a predetermined color on a region where the electrostatic latent image is formed, to develop the electrostatic latent image. The developing devices 140 are provided under the photosensitive belt 100. Also, a dry roller 180, which contacts a heat roller 190, heats the photosensitive belt 100. The roller is disposed parallel with the third roller 113, to dry the developer supplied from the developing devices 140.
A toner image developed in a predetermined region of the photosensitive belt 100 by the developing devices 140 is transferred onto recording paper 160 by a transfer roller 150 which is parallel with the first roller 111, and the photosensitive belt 100 passes between the first roller 111 and the transfer roller 150. The toner image transferred onto the recording paper 160 is fixed by the fixing device 170 to obtain a desired image.
FIG. 2 is a graph illustrating a conventional method for controlling the temperature of the fixing device of the above-described image forming apparatus. Referring to FIG. 2, when a power supply of the image forming apparatus is turned on at the time t.sub.0 the temperature of the fixing device 170 having an ambient temperature T.sub.A, is increased to a print waiting temperature T.sub.PW. At the time t.sub.1 printing data is input, and the temperature of the fixing device 170 increases to a printing temperature T.sub.P required for printing. At the time t.sub.2, the printing is completed, and the temperature of the fixing device 170 is lowered to the print waiting temperature T.sub.PW. A first timer installed in a central processing unit (not shown) for controlling the temperature of the fixing device 170 begins to operate at the time t.sub.2. When no printing data is input during the predetermined time interval (t.sub.3 -t.sub.2), set by a user, after the beginning of the operation of the first timer, the temperature of the fixing device 170 is lowered to an energy saving temperature T.sub.S at the time t.sub.3. Then, a second timer (not shown) installed in the central processing unit operates. When the printing data is not input during the time interval (t.sub.4 -t.sub.3), set by a user, after the beginning of the operation of the second timer, the fixing device 170 is turned off. Here, when the printing data is input at time t.sub.5, the temperature of the fixing device 170 is increased to the printing temperature T.sub.P. Then, at time t.sub.6, the temperature of the fixing device 170 is lowered to the print waiting temperature T.sub.PW.
Meanwhile, when the printing data is input during the operation of the first or second timer, the first or second timer is initialized. The temperature of the fixing device 170 is increased to the printing temperature T.sub.P, i.e., the temperature required for printing, and then the printing is performed. When the printing is completed, the temperature of the fixing device 170 is lowered to the print waiting temperature T.sub.PW, and the first or second timer operates again.
By the conventional method of controlling the temperature of a heating source, when the time interval (t.sub.3 -t.sub.2) or (t.sub.4 -t.sub.3) passes after the beginning of the operation of the first and second timer, respective, the temperature of the fixing device 170 is rapidly lowered. When the time interval (t.sub.3 -t.sub.2) or (t.sub.4 -t.sub.3) is shortened for the purpose of energy saving, it increases the likelihood that a user will have to wait for the fixing device to reach the printing temperature. This increases the amount of wait time for using the device to allow the fixing device to reach T.sub.P. On the other hand, when the time interval (t.sub.3 -t.sub.2) or (t.sub.4 -t.sub.3) is increased, unnecessary energy is consumed.